This invention relates to the measurement of road surface friction encountered by a vehicle skidding along a road, and more particularly to a method and an apparatus for obtaining data concerning the deceleration of a vehicle during a skid test and processing the deceleration data to determine such factors as the length of the skid and the coefficient of friction between the road surface and the skidding vehicle.
In a skid test, a vehicle is driven along a road at an initial speed, and then the brakes of the vehicle are applied so as to lock the wheels of the vehicle and cause it to skid to a stop. As is known, a vehicle skidding to a stop will leave skid marks, and the length of such a skid mark is indicative of both the speed of the vehicle when the skidding began and the road surface friction which is expressed as a coefficient of friction. However, during a skid test a vehicle also undergoes deceleration, and the rate of deceleration and the time required for the decelerating vehicle to skid to a stop from an initial speed are also indicative of the coefficient of friction and the length of the skid.
In the past, law enforcement agencies have utilized the skid marks of a motor vehicle involved in an accident to estimate a minimum speed that the accident vehicle was traveling when the skidding began. However, before estimating the speed of the accident vehicle it is first necessary to determine the coefficient of friction for the particular section of road where the accident occurred, and this is accomplished by performing a skid test over the particular road section. The speed of the test vehicle when same begins skidding, is often determined by a conventional radar speed detector, and upon measuring the length of the resulting skid marks the coefficient of friction may be calculated according to the formula:
Coefficient of Friction=(speed of test vehicle in miles per hour).sup.2 /F.times.(length of skid test in feet) wherein F as used herein is a constant equal to [(3600 seconds).sup.2 .times. miles.sup.2 .times.2.times.32 feet/second.sup.2 .times.hours.sup.2 .times.(5280 feet).sup.2 ] or [29.75 miles.sup.2 /hour.sup.2 -feet] or as is usually rounded to a value of 30.
However, in determining the coefficient of friction by a method utilizing the length of the skid marks, it is important to accurately located the point at which the test vehicle began skidding, and this is usually accomplished by having personnel along the side of the road to actually witness the lock-up of the wheels. Afterwards, the personnel must move onto the road to measure the length of the skid marks left by the test vehicle. Such a procedure is disadvantageous because of the amount of time it requires and the number of personnel involved in witnessing the wheel lock up. Moreover, since the skid test must be performed along the section of road where an accident occurred, the test necessarily involves stopping or diverting traffic from that area, and if traffic is allowed to resume on a limited basis while the skid marks are being measured, then those measuring the skid marks are exposed to potential traffic hazards.
In the past, apparatus has been developed for measuring the coefficient of friction without requiring personnel to witness a wheel lock-up or to measure the length of skid marks, but such measuring apparatus usually involves bulky and relatively expensive equipment in the form of an extra wheel or a trailer to be towed along the road behind a vehicle. Due to the bulkiness, such measuring apparatus is unsuitable for being carried as standard equipment in a law enforcement vehicle, and accordingly the measuring apparatus must be brought to the scene of an accident from a storage location which may be many miles away. This necessarily involves a delay in measuring the coefficient of friction, and a further delay may be incurred when the measuring apparatus is not readily available but perhaps is being used elsewhere. During such delays conditions may change on the road surface to substantially modify the coefficient of friction associated therewith. However, the principal factors affecting the value of the coefficient of friction at the scene of an accident may be the presence of ice, snow, water, debris or foreign particles on the surface of the road. When such factors may vary substantially in a relatively short period of time due to changes in the weather or the passage of traffic, it is desirable to minimize any delay in measuring the coefficient of friction.
It is further noted that the present invention may be utilized to appraise users of a roadway with information relevant to surface conditions, especially a relative braking efficiency as compared to a clean and dry surface. Such information may be provided to aircraft pilots landing on airport runways, automobile drivers during inclement weather, and the like.